The present invention relates in general to stereophonic radio receivers, and more specifically to a stereo blend controller for changing the stereo separation characteristic in response to the average RF environment at the receiving antenna.
FM stereo broadcasting involves the transmission of a left plus right (L+R) signal and a left minus right (L-R) signal. The left plus right signal is the only one recovered in monophonic receivers. Stereo receivers recover both the left plus right and the left minus right signals and then add these signals to form the left stereo signal and subtract the signals to form the right stereo signal.
In order to generate an acceptable stereo audio output, the RF broadcast signal must be received at a field intensity greater than some minimum field intensity (e.g., 30 .mu.V). For field intensities less than the minimum, only a monophonic signal is generated in the receiver in order to avoid the noise that would result from attempting to decode a weak stereo signal. For field intensities greater than the minimum but less than a higher field intensity (e.g., 300 .mu.V) where full stereo separation can be achieved, partial stereo (i.e., a blend of stereo and mono signals) is generated by the receiver.
Shifts in stereo separation greater than some particular level and at some particular rate of change can be heard by a listener and are to be avoided. Thus, it is advisable to vary the stereo blend gradually so as not to produce objectionable sounds.
Conventional receivers control stereo separation with circuitry in the stereo decoder for variably attenuating the L-R signal. The amount of stereo blend is determined by the field intensity at the antenna of the RF broadcast signal being received. A fixed relationship defines the magnitude of a blend control signal to the stereo decoder in dependence on the field intensity signal. Thus, there is no stereo separation if field intensity is below a first predetermined value, increasing separation as field intensity increases, and substantially full stereo separation if field intensity is above a second predetermined value.
The relationship between field intensity and the amount of attenuation of the L-R signal to achieve a desired stereo separation is known as a blend curve. Field intensity, or field strength, is the effective electric field strength caused by a radio wave at a point in the path of the radio wave. Under certain conditions, such as the occurrence of multipath distortion, field intensity fluctuates rapidly. Multipath distortion is caused when signals following direct and indirect (e.g., reflected) paths from the transmitter to a receiver interfere at the receiver. The blend curve includes a gradual slope so that field intensity fluctuations from multipath distortion do not cause rapid variations in the stereo separation, thereby avoiding phase distortions in the audio output heard by the listener. Nevertheless, the blend curve must reach full separation by some level of field intensity and cannot provide a shape having only a gradual slope for all field intensities.
The fixed blend curve of the prior art is optimized to some model of typical receiving conditions in terms of normal field intensity and severity of multipath. Thus, a blend curve may be optimized for an urban area characterized by strong field intensities (because of the proximity to the transmitting antenna) and by strong multipath (because of reflections by buildings). For these assumed conditions, a blend curve can be shaped such that the blend control signal operates most of the time at a portion of the blend curve having a gradual slope. Stereo performance is then less than optimal for other conditions, however. For received signals having a different average field intensity, stereo blend may be operating at a point in the blend curve not having a gradual slope so that multipath does cause phase distortions. Furthermore, optimizing a blend curve for high field intensities can result in suppressing the reception of stereo at field intensities that while they are lower could still support stereo reception.